(1) Field of the Invention
The present invention relates to a process for the preparation of a sintered (dense) silicon nitride body excellent in the strength at high temperatures. More particularly, the present invention relates to a process for preparing a sintered silicon nitride body having an improved surface texture by hot isostatic pressing. Furthermore, the present invention relates to a novel sintered silicon nitride body prepared according to this process.
(2) Description of the Prior Art
Recently, the HIP process (hot isostatic pressing) has attracted attention as means for sintering a ceramic material to full density with use of a small amount of sintering aids, and for treating various materials. Especially for obtaining a sintered body having a complicated shape from a molded powder, there is mainly adopted a so-called glass capsule process in which powder or powder compact, a green molded body or a pre-sintered body is filled in a glass (Vycor or Pyrex) vessel and is preliminarily heated under compression by an inert gas at a temperature close to the softening point of the glass vessel to cover the periphery of the compressed body with the glass, the interior powder, green molded body or pre-sintered body is densified in a high-temperature high-pressure gas by using the covering glass layer as a pressure-transmitting medium, and the intended sintered body is obtained by removing the glass from the periphery of the formed sintered body by using a dissolving solution or with the sand blasting method.
This process, however, is defective in that since a large quantity of the softened glass component intrudes in the grain boundary phase of the sintered body, the strength at high temperatures is drastically reduced.